This invention relates to the provision of optical coupling between optical components, and in particular to the provision of optical coupling between one or more optical components, at least one of which is an optical waveguide, and one or more other optical components that may for instance be constituted by electro-optic transducers or passive optical components such as micro lenses or other waveguides.
The precision of alignment that is required between the end of an optical waveguide, and that of some other optical component with which that waveguide is required to be optically coupled, depends upon the particular application involved. Thus relatively little precision is required in aligning an optical fibre with a slow-speed large-area photodiode, whereas somewhat greater precision is generally required for aligning a multimode fibre with a light emissive diode, and still greater precision is typically required for aligning a single mode fibre with an injection laser. Alignment between components may be achieved using either a dead-reckoning approach or a monitored approach.
In the case of a dead-reckoning alignment approach, the components to be aligned are placed in intimate contact with some alignment substrate whose shape is such that the mere placing of the components in such intimate contact automatically serves to locate them with the necessary precision relative to each other. All that is then necessary is to retain the components in position on the alignment substrate, and this can often conveniently be accomplished by the application of some fixative medium to lock the components in place. This form of dead-reckoning alignment, using photolithographically etched single crystal silicon alignment substrates, has for instance been demonstrated to provide adequate precision for aligning multimode fibres with injection lasers as described by K P Jackson et al in a paper entitled, `Optical Fiber Coupling Approaches for Multichannel Arrays`, SPIE Vol. 994, Optoelectronic Materials, Devices, Packaging And Interconnects II (1988) pages 40-47. The authors of the paper state that their photolithographic processing enables them to position the end of a 50/125 .mu.m graded-index multimode fibre relative a laser with a positional accuracy of .+-.31 .mu.m. In the corresponding case of positioning the lensed end of a standard single mode fibre relative an injection laser designed for direct modulation it is typically desirable to keep the coupling efficiency to within 1 dB of its maximum value and this typically requires a precision of about .+-.0.7 .mu.m in directions at right angles to the fibre axis. The precision requirements are even greater for coupling a wedge-ended single mode fibre with the more highly astigmatic emission from a 980 .mu.m wavelength pump laser, for which the equivalent precision requirement is about .+-.0.41 .mu.m.
In order to satisfy these more stringent precision requirements, recourse is typically had to some form of monitoring in which the laser is energised, and the coupling efficiency actually achieved is monitored, with the aid of a photodetector, while the alignment between laser and fibre is adjusted so as find the alignment position providing optimum coupling efficiency. Once that alignment position has been found, the alignment should be held steady while a bonding agent is applied to provide permanent fixing. This type of alignment procedure that involves monitoring will hereafter be referred to as active alignment, whereas the dead-reckoning alignment procedure to which previous reference has been made will be referred to as passive alignment.
A significant problem which is often encountered with active alignment is the tendency of the bonding agent to disturb the alignment as it effects the bond. Thus if a resin bonding agent, such as an epoxy resin, is used, it is found that the shrinkage which occurs on curing of the resin is very liable to disturb the alignment. A similar effect is also encountered when using solder or brazing metal as the bonding agent, or when using welding such as laser beam welding to effect the bond. In each case of soldering brazing or welding, shrinkage occurs during the freezing of the molten metal. Such misalignment due to shrinkage of bonding materials is for instance mentioned in GB-A-2 146 841. The coupling arrangement may be such as to allow for this type of misalignment to be compensated by subsequent plastic deformation of the supporting structure for the fibre, but this approach to the solving of the problem is liable to introduce longer term instability problems associated with the relaxation of residual stresses introduced into the structure during the plastic deformation procedure. Such relaxation may be induced by thermal cycling of the coupling. Thermal cycling can also induce phase separation in solders, particularly high lead content solders, that can induce relaxational type movement.